Method and apparatus for providing a very fast acting noise squelch control system for an RF radio receiver

ABSTRACT

A fast acting noise squelch control circuit for FM radio receivers samples the noise above the audio band to determine the presence of an RF carrier. The detected noise signal is integrated by a low-pass frequency gain integrating filter and compared, using a high gain differential amplifier, against a preset squelch threshold voltage to provide a signal indicative of the presence or absence of a received carrier. The output of the integrating filter is initialized at a reference voltage that is only slightly below the threshold voltage setting for the comparator to minimize integration time and increase sensitivity of the squelch. In this manner, a long time constant in the noise integrating filter can be used without having to wait for a long time to establish whether a carrier signal is present. After a predetermined observation interval, the comparator output is checked and the comparator threshold voltage setting is lowered if a carrier of sufficient signal strength has been detected to increase the squelch control circuit hysteresis. The improved squelch control system can be utilized in intermittent or scanning receiver systems while using the normal noise filtering as would be employed in continuously operated receivers without having to use a long &#34;turn-on&#34; period that would result in unacceptably slow cycling rates.

FIELD OF THE INVENTION

The present invention relates generally to squelch circuitry used inradio receivers to eliminate noise between intervals of signaltransmission. More particularly it relates to an improved squelchcircuit wherein the squelch turn-on delay time is significantly reducedwithout requiring a large amount of squelch threshold hysteresistypically necessary in fast acting squelch circuits. The improvedsquelch circuit is especially applicable to FM receivers operating in apower saving or intermittent mode and can also be used advantageously inscanning receivers where it is desirable to have the squelch operate asrapidly as possible.

BACKGROUND OF THE INVENTION

In radio receivers, particularly in mobile and high gain FMcommunication receivers, it is well known to utilize squelch circuits toautomatically cut off the audio output in the absence of a receivedcarrier in order to prevent annoying receiver noise from being audibleduring intervals between signal reception. Upon the reception of areasonable strength carrier signal following an interval of silence, thesquelch circuit reactivates the audio output, allowing the signal to beheard as long as it is present. However, prior art squelch circuits forcontrolling the audio output of radio receivers, particularly FMscanning receivers, typically suffer from the problem of having longturn-on and turn-off delay times. This can produce an initial loss ofaudio output upon reception of weaker transmissions and allows anannoying burst of noise or "squelch-tail" at the end of a transmission.Although squelch circuits having a very short time constant could beused, they are problematic because the squelching operation of thereceiver then becomes overly sensitive to any temporary or rapidvariations in the strength of the received signal, commonly referred toas "flutters" or "fades", which cause the squelch to produce annoyingintermittent interruptions in the audio output.

In a scanning receiver or in "priority search" type applications, aconventional noise squelch system would discriminate against low signallevel transmissions, since the squelch operation is too sluggish,whereas a very fast acting squelch would require a large amount ofhysteresis for satisfactory operation. It is thus desirable to provide afast acting squelch circuit which does not interrupt the audio duringtemporary fluctuations in signal strength due to flutters or fades andwhich also prevents the occurrence of a long squelch tail at the end ofstrong transmissions. The present invention addresses these problems andis particularly applicable toward achieving fast squelch operation in FMreceivers operating in an intermittent mode (e.g., a power saving mode)or in scanning receivers.

In FM receivers, the receiver noise above the audio band is commonlyused to indicate the presence of a carrier signal. A suitable thresholdvalue for the noise level is established for a receiver (depending onits particular signal to noise ratio characteristics) and is used todetermine when to enable or disable the receiver's audio output. A blockdiagram of a conventional prior art noise squelch system of this sort isillustrated in FIG. 1. Typically, the above audio band receiver noise isfiltered to remove the lower frequency speech components, then amplifiedand rectified. This detected noise voltage is subsequently integratedand the integration output is then compared with a DC squelch thresholdreference voltage by a comparator. The decision output of the comparatoris then used to control a muting switch in the audio output path.

Referring to the prior art example illustrated in FIG. 1, the signalfrom a radio receiver FM demodulator 10 is typically filtered by ahigh-pass filter or a band-pass filter 12 to extract the above-audioband noise. This noise signal is amplified by the noise amplifier 14 andrectified by noise rectifier 16 to produce a DC voltage output. Thisdetected noise voltage can be used as a measure of the strength of areceived FM carrier signal. If the detected voltage is directly relatedto the above audio band noise level, then it will be inversely relatedto the received FM signal strength. Accordingly, noise rectifier 16 isoften designed to produce a detected voltage that is inversely relatedto the noise level so that it will be directly related to FM signalstrength. A prior art squelch circuit of this general sort is disclosedby U.S. Pat. No. 4,359,780 to Day (1982).

In the FIG. 1 depiction of a typical prior art circuit, the DC voltageoutput from noise rectifier 16 is applied to input 17 of squelch controlcircuit 20. Within squelch control circuit 20, the detected noisevoltage is filtered to eliminate low-frequency DC fluctuations andintegrated by low-pass filter/integrator 18. The integrated signal iscompared to a predetermined squelch threshold voltage by comparator 19,and the decision output 11 is used to indicate RF carrier activity.

Additional exemplary prior art squelch circuit configurations aredisclosed in the following U.S. Patents:

U.S. Pat. No. 3,769,592 to Espe (1973)

U.S. Pat. No. 3,979,679 to Bush et al (1976)

U.S. Pat. No. 4,085,370 to Vanderpole et al (1978)

U.S. Pat. No. 4,132,953 to Martin, III (1979)

U.S. Pat. No. 4,176,286 to Shuffield, Jr. (1979)

U.S. Pat. No. 4,479,250 to Flood (1984)

U.S. Pat. No. 4,731,868 to Dreier (1988)

U.S. Pat. No. 4,947,456 to Atkinson et al (1990)

U.S. Pat. No. 4,972,510 to Guizerix et al (1990)

Usually, the comparator in a squelch control circuit is designed withsome positive feedback to minimize any "dithering" of the squelch outputabout a pre-set reference threshold noise level caused by fluctuationsin the rectified detected noise output. This is accomplished byproviding hysteresis in the setting of the squelch comparator thresholdlevel so that the receiver will unsquelch at a higher level input RFsignal strength than it takes to squelch it. However, in order for asquelch circuit to operate as rapidly as possible, it is essential thatthe filtered and integrated output of the noise rectifier reach itsfinal value quickly. This implies using a relatively wide bandwidthlow-pass filter with the filter/integrator 18 in the squelch controlcircuit. Unfortunately, the use of a relatively wide bandwidth filteralso inherently allows relatively large fluctuations in the integratorDC output. This necessitates designing the squelch circuit comparatorwith a large amount of hysteresis to keep the squelch from "bobbling"around a single set reference threshold noise level. However, a largeamount of hysteresis is undesirable since it reduces the effectivesensitivity of the receiver and allows annoying squelch-tail noisebursts at the end of transmissions and, in a scanning type of receivercontext, may cause one to entirely bypass lower level RF carriers.

BRIEF SUMMARY OF THE INVENTION

The present invention circumvents these problems by taking advantage ofcertain known and predictable characteristics of FM receivers and, inparticular, FM radio receivers utilizing a frequency synthesizer andcapable of operating in an intermittent or power saving mode or in ascanning mode. More specifically, in such receivers, there exists apredictable "turn-on" delay period before the receiver is able toproduce a usable signal because the receiver's frequency synthesizerrequires a short period to "lock on" to a selected channel. The durationof this initial turn-on delay is usually known or predicable for aparticular receiver design. Accordingly, in the present exemplaryembodiment of this invention, a signal path to the filter is establishedonly after expiration of this known turn-on delay period, when a usefulsignal input to the FM demodulator can be expected. Additionally, sincethe threshold setting of the noise comparator is a predeterminedparameter, the delay incurred by the integrator output in reaching thatpreset level can be substantially reduced by biasing or initializing theintegrator output close to (e.g., just below) that threshold level. Thenoise comparator decision output then can be sampled after a muchshortened interval--e.g., commensurate only with the time constant ofthe integrator's low-pass filter and its initialized pre-biased outputlevel. The comparator threshold level may then subsequently be changed(depending on its sampled output) to dynamically adjust the amount ofdecision hysteresis required in the functioning of the squelch for aparticular receiver.

In accordance with a preferred exemplary embodiment of the presentinvention, the establishment of a rectified noise signal path, theinitializing of the integrator output and the changing of the noisecomparator threshold level are each accomplished under microprocessorcontrol via electronic switches, (e.g., MOS-transconductance gates). Thegeneral use of microprocessor controllers in FM radio receivers forperforming various switching, timing and control functions is itselfknown in the art (e.g., see U.S. Pat. No. 4,947,456 to Atkinson et al).Accordingly, the additional programming of a microcontroller toaccomplish the few added switching and timing functions of the presentinvention (e.g., as a part of the typical signal acquisition or controlmodule of such computer program) would be well within the level ofordinary skill in this art. Alternatively, the squelch control switchingsignals for implementing the above functions could be generated by usingdigital logic and timing circuits comprising conventional modules suchas "one-shots" or monostable multivibrators formed from circuits in anyform (e.g., as discrete circuits, as part of larger integrated circuits,etc.)

Briefly described, the invention provides an improved fast acting noisesquelch control circuit for FM radio receivers. The input to the squelchcontrol circuit is obtained from a noise detector circuit that isresponsive to high frequency noise above the audio frequency band (e.g.,6 kHz to 25 kHz). The noise input signal is integrated by a limitedlow-frequency gain operational amplifier integrator and compared, usinga high gain differential amplifier comparator, against a preset squelchthreshold voltage to indicate the presence or absence of a receivedcarrier. In this manner, the output of the comparator provides a carrieractivity signal that is used to control muting of the receiver audiooutput. In the improved squelch control circuit of the presentinvention, the integrator output is initialized at a voltage level thatis only slightly below the preset squelch threshold voltage setting forthe comparator to minimize integration time delay. If an FM carrier ofsufficient signal strength is detected beyond a predetermined period oftime, the amount of squelch hysteresis is then subsequently increased bylowering the squelch threshold voltage setting of the comparator.

BRIEF DESCRIPTION OF THE DRAWINGS

These as well as other objects and advantages of this invention will bemore completely understood and appreciated by carefully studying thefollowing detailed description of a preferred exemplary embodiment takenin conjunction with the accompanying drawings, of which:

FIG. 1 is a block diagram representing a conventional prior art noisesquelch system for an FM receiver;

FIG. 2 is a schematic diagram of a fast acting squelch control circuitin accordance with an embodiment of the present invention;

FIG. 3 is a relational timing diagram for the states of the controlledswitches and the carrier activity sensor output of the comparator in thesquelch control circuit of FIG. 2;

FIG. 4 is a flowchart of an exemplary program executed by themicrocontroller of FIG. 2 for controlling the operation of the squelchcircuit in accordance with an embodiment of the present invention; and

FIG. 5 is a schematic diagram of an alternate embodiment for a squelchcircuit switch controller.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 2, there is illustrated in an improved squelch controlcircuit 20' for an FM radio receiver. This circuit can be utilized withany communications receiver containing a wide band discriminator andcapable of sampling the noise above the audio band to determine thepresence of an RF carrier. The detected noise signal, V_(in), from theoutput of the noise rectifier 16 is applied to the inverting input 28 ofoperational amplifier (op-amp) U1 via resistor R1. With the parallelcombination of resistor R4 and capacitor C1 in a feedback path fromoutput to inverting input, op-amp U1 functions as a limitedlow-frequency gain integrator and, thus, also operates as a low-passfilter. An initializing reference voltage, V_(ref), is provided to thenon-inverting input 29 of op-amp U1 by the voltage divider configurationof resistors R2, R3, and R5. In order to initialize the output voltage,V_(out), of op-amp U1 to V_(ref), (i.e., to effectively eliminateunnecessary time delay in getting the filter output up to a valuenominally commensurate with the noise signal as soon as it becomesavailable), a controllable switch S1 is coupled in parallel withresistor R4 and capacitor C1. For the particular circuit arrangementdepicted in FIG. 2, the output of op-amp U1 is given by the followingequation: ##EQU1##

The transfer function, V_(out) /V_(in), for the limited low-frequencygain integrator, U1, is of the form of a simple one pole RC low-passfilter, as follows: ##EQU2## Where, ##EQU3##

T=C1.R4

s=jw

In a preferred exemplary embodiment of the present invention, switch S1is controlled by microprocessor 50. When S1 is closed, the output ofop-amp U1 is shorted to its inverting input 28 and V_(out) becomes equalto V_(ref) regardless of the input signal, V_(in') from noise rectifier16 (i.e. if R4=0, then V_(out) =V_(ref)). If the average output voltagefrom noise rectifier 16 equals V_(ref), then the DC output of integratorU1 equals V_(ref). However, when the rectified noise V_(in) at input 17exceeds V_(ref), then the output voltage V_(out) of integrator U1 willbe below V_(ref) and vice versa. If S1 is closed, the output ofintegrator U1 is held at V_(ref), independent of the signal level atinput 17.

Referring again to FIG. 2, the output of integrator U1 is coupled to theinverting input 42 of op-amp U2 which functions as a comparator. Op-ampU2 may be any type of general high gain differential amplifier. Thenon-inverting input 43 of op-amp 40 is coupled to the common junction ofresistors R3 and R5 via resistor R6 to provide an initial predeterminedsquelch noise level threshold reference voltage, V_(ST), for comparatorU2 which is slightly greater than the reference voltage, V_(ref') usedto initialize the output of integrator U1. The non-inverting input 43 ofop-amp U2 is also coupled to controllable switch S2 via resistor R7.Switch S2 is coupled between resistor R7 and a circuit ground and isalso controlled by microcontroller 50. The output of comparator U2 iscoupled to a sampled data input port of microcontroller 50 and providesa signal to a Carrier Activity Sensor (CAS) input function of theprogrammed microcontroller that indicates whether the detected noisesignal at squelch control circuit input 17 is above or below the squelchnoise threshold reference level established by V_(ST). Microcontroller50 samples the signal on the CAS input at desired times (e.g., initiallyat the predetermined time a useful CAS signal is first expectedfollowing receiver turn-on or carrier lock-on) and uses its status toimplement audio muting in the receiver and control squelch levelhysteresis via switch S2.

When the radio receiver is first powered up or when a scanning receiveris acquiring a new channel, switch S1 is initially maintained in theclosed position and switch S2 is kept open. Consequently, the output ofU1 is initialized at V_(ref). Since the initial squelch noise levelthreshold voltage, V_(ST), at comparator U2 is slightly higher thanV_(ref), the output on line 41 of comparator U2 is high, indicating a"no carrier present" condition. After an initial predetermined delayperiod allowing the frequency synthesizer of the receiver time toproperly lock on frequency, the receiver is able to provide a meaningfuldetected noise output from noise rectifier 16. Switch S1 is then openedby a signal from microcontroller 50 on line 51. During this time, switchS2, also controlled by microcontroller 50 via line 52, is maintained inthe open position, assuring no immediate change in squelch noise levelthreshold voltage V_(ST).

When switch S1 is opened, the detected noise voltage at squelch controlcircuit input 17 is integrated by U1 and the integrator's outputvoltage, V_(out), will begin to ramp either up or down, depending onwhether the average detected noise at input 17 is below or aboveV_(ref). If a sufficiently high level RF carrier signal is beingreceived (i.e., no above-audio band noise signal detected), outputvoltage V_(out) of integrator U1 will rise above squelch noise levelthreshold voltage V_(ST) and the output signal of comparator U2 on line41 will go low. Since the output of integrator of U1 is initialized at alevel that is already close to the comparator threshold, a long RC timeconstant can be used in implementing the noise integrating filterwithout experiencing an excessive integration delay time to establishwhether a carrier signal is present.

At the end of a predetermined observation interval, during which thedetected noise signal is continuously integrated, microcontroller 50checks the CAS input line 41 to determine the output decision status ofcomparator 40. If line 41 is low, the microcontroller 50 closes switchS2 via switch control signal line 52. Once switch S2 is closed, theinitial squelch noise level threshold voltage, V_(ST), is lowered by thevoltage divider combination of resistors R6 and R7. This providessquelch decision hysteresis for squelch control circuit 20 in theoperation of comparator U2 by assuring that the CAS input line 41 willstay low unless the output of noise integrator U1 declines below thisnew lower threshold level. Thus, the detected noise voltage at input 17must now increase materially before the signal on CAS input line 41 willgo high indicating a need to squelch the receiver audio output.

Accordingly, if the CAS input line 41 is low at the end of theintegration interval, indicating an adequate RF carrier signal input tothe receiver, the receiver audio is left turned on until the new noisethreshold is exceeded. The fact that the microcontroller checks the CASinput line only after a predetermined observation interval, assures thatno premature noise level determination is made despite the close initialproximity of integrator output voltage V_(out) to the decision thresholdvoltage level, V_(ST), of comparator U2. At the end of a receivedtransmission, the detected noise will exceed the new threshold. SwitchS2 is then opened, and switch S1 is closed to reinitialize the squelchcontrol circuit.

FIG. 3 depicts a generalized timing diagram showing the output ofintegrator U1, the output of comparator U2 and the relative on and offstatus for the controlled switches S1 and S2, in FIG. 2. Switch S1 iskept closed during an initial delay interval from T₀ to T₁ to allow thereceiver time to settle and lock on frequency. After an integrationobservation interval from T₁ to T₂, the output of comparator U2 ismonitored by microcontroller 50 to determine carrier activity status.Depending on the status of CAS input line 41, the initial squelch noiselevel threshold voltage V_(ST) is lowered (e.g., to V_(STL)) at time T2by closing S2 or the circuit is reinitialized before the receiveracquires a new channel frequency. The squelch control circuit is alsoreinitialized if the received carrier signal strength causes theintegrated noise output of U1 to fall below the lowered thresholdvoltage, V_(STL), at a time later then T₂ ;for example, at a time T₃ +Δ.The squelch control circuit will not be reinitialized, however, fortemporary or rapid "fades" in received carrier signal strength which donot cause the integrated noise output of U1 to fall below V_(STL), asfor example, at time T₃.

FIG. 4 depicts a flowchart for a typical program module to be executedperiodically by microcontroller 50 for controlling the timing and on/offstates of switches S1 and S2 for proper operation of the squelch controlcircuit 20' When the receiver is initially powered up or when switchingto a new channel, the squelch circuit control program is entered at step60. Switches S1 and S2 are assumed to have been initialized in closedand open conditions at some earlier time (e.g., by an initialinitialization at operator turn-on of the entire system or by an initialpass through the module of FIG. 4). If the initial conditions (e.g., S1closed) are detected at decision point 62, then switch S1 is openedafter a short receiver settling delay at step 64. A predeterminednominal integration interval is observed at step 66 before the CAS inputis monitored at decision step 68. Depending on the status of the CASinput, S2 is closed to lower the squelch comparator threshold andintroduce hysteresis at step 70 during unsquelched reception or thecontrol circuit is reinitialized at step 69 before return to a maincalling program at 72. Alternatively, if the program segment of FIG. 4is entered for a periodic check of CAS during previously unsquelchedreception, then decision step 62 will cause control to skip immediatelyto the CAS check at 68.

FIG. 5 depicts an alternate embodiment for a switch controller for thesquelch circuit of FIG. 2. In this embodiment, a hardware controller 80is provided as separate from a primary receiver controller circuit 82.Squelch controller 80 consists of a "one-shot" or monostablemultivibrator 83 and controllable switch S3. Whenever the receiver isturned on or switched to a new channel, one-shot 83 is triggered by asignal from receiver controller 82 via line 81. This results inmaintaining S1 closed until one-shot 83 times out. Accordingly, theoutput of U1 remains at V_(ref) until the receiver is on frequency.

Referring again to FIG. 3, after one-shot 83 times out and switch S1opens at time T₁, the output of U1 is allowed to settle. A predeterminedsettling period later, at time T₂, receiver controller 82 activatescontrollable switch S3 via line 84 to permit CAS output line 93 tocontrol switch S2 in the squelch circuit. At some later time, T₂ +Δ, ifthe CAS output is still low, the controller 82 will keep the receiverturned on. However, when CAS goes high (e.g., at the end of atransmission), the receiver reverts to a standby mode where it is onlyperiodically turned on to see if a carrier signal is present. However,for a scanning receiver or for a priority search application, thereceiver is permanently turned on but "peeks" at a priority channel tocheck if that channel is active by changing channels momentarily.

Although a few exemplary embodiments of the present invention have beendescribed in detail, it is to be understood that the invention is notlimited to the disclosed embodiments, but also may be modified while yetretaining one or more novel advantages. All such modifications andequivalent arrangements are intended to be included within the spiritand scope of the appended claims.

What is claimed is:
 1. A fast acting noise squelch control system for aradio receiver operable in response to a detected noise voltage,comprising:an integrating filler responsive to high-frequency noise forremoving low-frequency voltage fluctuation from the detected noisevoltage and for integrating the detected noise voltage; a first switchcoupled between an input and an output of said integrating filter forinitializing said filter output to a first reference voltage; acomparator, including a reference input, coupled to said integratingfilter for comparing an integrated noise signal to a second referencevoltage, applied to said reference input, and for generating a carrieractivity signal when said integrated noise signal exceeds said secondreference voltage, a second switch, responsive at least in part to saidcarrier activity signal and coupled between said reference input of saidcomparator and a circuit ground, for changing the value of said secondreference voltage to dynamically alter a hysteresis of thesquelch/unsquelch operation in said radio receiver; and a radio receivercontroller circuit connected to receive said carrier activity signalfrom said comparator and to squelch or unsquelch audio circuits of saidradio receiver in response thereto.
 2. A fast acting noise squelchcontrol system as in claim 1, further comprising:a resistor dividernetwork coupled between a voltage source and said integrating filter andsaid comparator for providing said first reference voltage to saidintegrating filter and said second reference voltage to said comparator.3. A fast acting noise squelch control system as in claim 2, whereinsaid first reference voltage is less than said second reference voltage.4. A fast acting noise squelch control system as in claim 3, whereinsaid first and second reference voltages are nearly equal.
 5. A fastacting noise squelch system as in claim 1, wherein said comparatorincludes a high gain differential operational amplifier.
 6. A fastacting noise squelch system as in claim 5, further comprising anadditional resistor coupled between a noninverting input of saiddifferential operational amplifier and said second switch for providinga third reference voltage to said comparator when said second switch isclosed.
 7. A fast acting noise squelch system as in claim 1, whereinsaid integrating filter includes an operational amplifier having aresistor and capacitor coupled in parallel between an output and aninverting input of said operational amplifier.
 8. A fast acting noisesquelch system as in claim 1, further comprising a microcontroller formonitoring an output of said comparator and for generating controlsignals for changing the state of said first and second switches.
 9. Amethod for providing a fast acting noise squelch operation for an FMradio receiver having an above-audio band noise detector and a squelchcontrol circuit comprising a noise integrating filter and a comparator,comprising the steps of:initializing the output of the integratingfilter at a first reference voltage; providing the comparator with asecond reference voltage that is slightly greater than said firstreference voltage; detecting above-audio band noise received by saidradio receiver by said above-audio band noise detector; integrating anabove-audio band noise signal from said noise detector by said noiseintegrating filter to produce an integrated noise signal; comparing saidintegrated noise signal at the output of said integrating filter withsaid second reference voltage by said comparator; providing a signalindicative of the presence of a received RF carrier in response to theoutcome of said comparing step; and determining whether squelching ofthe radio receiver output is desired in response to said carrierindicative signal.
 10. A method for providing a fast acting noisesquelch as described in claim 9, further comprising the step of:changingsaid second reference voltage to a third reference voltage value that issignificantly lower than either said first or second reference voltagesafter the presence of a carrier has been indicated for a predeterminedperiod of time.
 11. A method for providing a fast acting noise squelchas described in claim 9, further comprising the step of:delaying saidintegrating step for a short predetermined interval after saidinitializing step to allow the receiver to lock on to a particularreceived signal.
 12. A fast acting squelch control system for an RFradio receiver, said system being operable in response to detected noisesignals on a given RF channel and comprising:a noise signal integratingcircuit, receiving a channel having an output which is preset to apredetermined initial value until the RF radio receiver has settled ontoa newly tuned RF channel and which is thereafter released to followdetected noise signals present on such channel; a noise level comparatorcircuit connected to compare the output of said noise signal integratingcircuit to a threshold value which is initially set to a level having aknown proximate relationship to said predetermined initial value andwhich said threshold value is reset to a substantially lower value apredetermined time interval after said noise integrating circuit presetoutput is released, establishing a predetermined level of operatinghysteresis thereafter; and a squelch control circuit connected toreceive the output of said comparator and to squelch or unsquelch audiocircuits of said RF radio receiver in response thereto.
 13. The squelchcontrol system as set forth in claim 12, wherein said threshold valueprovided to said comparator is initially set to a level slightly greaterthan said predetermined initial value of said integrating circuit. 14.The squelch control system as set forth in claim 12, further comprisinga microcontroller means for automatically resetting said threshold valuein response to a predetermined output condition of said comparatorcircuit so as to dynamically effect a predetermined noise sensitivityhysteresis response for said radio receiver.
 15. In a method forproviding a fast acting squelch control for an RF radio receiver whereinsaid squelch is controlled in response to detected noise signals on agiven RF channel, an improvement comprising the steps of:presetting theoutput of a noise signal integrating circuit to a predetermined initialvalue; applying a channel to said noise signal integrating circuit;maintaining the output of the integrating circuit at said initial valueuntil the RF radio receiver has settled onto the newly tuned RF channel;releasing the output of the integrating circuit from said initial valueafter the radio receiver has settled onto the newly tuned RF channel toallow said output to follow detected noise signals present on suchchannel; and determining whether to squelch or unsquelch the radioreceiver in response to the output from said noise signal integratingcircuit being above or below a predetermined level.
 16. In a method forproviding a fast acting squelch control for an RF radio receiver whereinsaid squelch is controlled in response to detected noise signals on agiven RF channel, an improvement comprising the steps of:integratingsaid detected noise signals; comparing said integrated noise signals toa set threshold value; automatically resetting said threshold value to asubstantially lower value at a predetermined time interval after saidintegrating of noise signals has begun so as to establish apredetermined level of operating hysteresis thereafter; and squelchingor unsquelching audio circuits of said RF radio receiver in response tothe results of said comparing step;wherein the integrating of detectednoise signals is initialized at a predetermined integration referencevalue and the integrated noise signals are first compared to said setthreshold value, and wherein said set threshold value is initially setto a level having a known proximate relationship to said initialintegration reference value.
 17. A method for providing a fast actingsquelch control for an RF radio receiver, said squelch controlled inresponse to detected noise signals on a given RF channel, comprising thesteps of:presetting the output of a noise signal integrating circuit toa predetermined initial value; maintaining the output of the integratingcircuit at said initial value until the RF radio receiver has settledonto a newly tuned RF channel; releasing the output of the integratingcircuit from said initial value to allow said output to follow detectednoise signals present on such channel; comparing the output of theintegrating circuit to a threshold value which is initially set to alevel having a known proximate relationship to said predeterminedinitial value; resetting said threshold value to a substantially lowervalue at a predetermined time interval after said noise integratingcircuit output is released so as to establish a predetermined level ofoperating hysteresis thereafter; and squelching or unsquelching audiocircuits of said RF radio receiver in response to the results of saidcomparing step.
 18. A method for providing a fast acting squelch controlas described in claim 17, wherein the result of said comparing step ismonitored by a microcontroller and said threshold value is conditionallyreset to said substantially lower value depending on a predeterminedresult of said comparing step.
 19. A method for providing a fast actingsquelch control as described in claim 13, further comprising the stepsof:integrating said detected noise signals; comparing said integratednoise signals to a set threshold value; resetting said threshold valueto a substantially lower value at a predetermined time interval aftersaid integrating of noise signals has begun so as to establish apredetermined level of operating hysteresis thereafter; and squelchingor unsquelching audio circuits of said RF radio receiver in response tothe results of said comparing step.